This invention relates to an enthalpy control system for conserving energy in an air conditioning system. More particularly, this invention relates to a self-contained sensing unit for measuring the wet bulb temperature of outside and return air under conditions which approximate adiabatic saturation and provide an output signal representing such wet bulb temperature for use in such a control system. Still more particularly, this invention relates to a control system for coordinately controlling return air dampers, outside air dampers and air conditioning components, including means for chilling water, to conserve energy based on the optimum mixture of return air, outside air, and chilling effects.
Special emphasis today is needed and is being placed on energy conservation. In any type of air conditioning system, it is less energy consuming to use outside or outdoor air whenever the enthalpy, or total heat, of the outdoor air is less than that of the air returning to the air conditioning system. While air conditioning systems sometimes have economizer cycles whereby the controls can automatically blend outside and return air for cooling during intermediate periods when the outdoor air is sufficiently cool, such systems, however, have utilized dry bulb measuring and control techniques. Moreover, historically, industrial heating and air conditioning systems have been switched from refrigeration or cooling to outside air control either manually or by an outdoor dry bulb thermostat. Manual operation is inefficient, attention-requiring, and often unrelated to the most energy conservative utilization of the system. On the other hand, any method of switching which relies on wet bulb sensing is apt to be inaccurate, approximate conditions of adiabatic saturation are deviated from, and also unrelated to the true total content of the outside and return air respectively.
The true measure of the enthalpy in air is obtained by observing the wet bulb temperature. Traditionally, in this industry, the wet bulb temperature is obtained by a conventional dry bulb thermostat having its sensing element surrounded by a wetted cloth or wick. This type of device has posed considerable maintenance problems inasmuch as the wick can be fouled by contaminants, including dirt, in the outdoor or return air. Moreover, the wick can become dry as a result of poor maintenance of the wetted wick and of a suitable air velocity. Thus, this type of instrument under those conditions began to operate like a mere dry bulb instrument and the benefit of enthalpy sensing no longer existed. Thus, in the industry, the aforementioned problems led to the adoption of conventional dry bulb changeover devices.
With the increased sensitivity to the need to conserve energy and in the face of rising power costs, it is a current problem in this art to produce a unitary, reasonably-priced enthalpy sensing device and an enthalpy-controlled control system using such a device to cause an air conditioning system to switch to outside air whenever the enthalpy of the outdoor air falls below the enthalpy of the air conditioned space. By the apparatus and control system of this invention, considerable operating energy of the refrigeration or air conditioning equipment can be saved.
For example, by reference to a psychometric chart, an assumed space design condition of 75.degree. F. dry bulb, 50 per cent relative humidity, and a design supply air temperature of 55.degree. F. dry bulb represents a wet bulb temperature of 62.3.degree. or an enthalpy of approximately 28 BTU per pound of dry air. Under a reduced outside air load, it is thus possible to cool the space effectively by a supply air temperature of 58.degree. F. or even higher, depending on the internal heat released from lights or other heat generating equipment in the space.
Accordingly, whenever the outdoor air enthalpy is lower than the return air enthalpy within the air conditioned space, it is more economical to cool 100 per cent outside air than to recirculate a mixture of outside air and room air. In addition, the use of increased proportions of outside air provides for a greater dilution of space odors and improved ventilation. As the enthalpy of the outdoor air reduces, greater savings in refrigeration power are thus achieved. At a predetermined point, the system can also be switched to a cycle whereby the outside and return air dampers are modulated to maintain the desired dry bulb temperature within the facility.
If an air washer system is utilized, even further savings can result to the consumer and less energy used as a result of the adiabatic saturation process which occurs in an air washer with no active refrigeration.
It is thus a principal object of this invention to provide a unitary, convenient, enthalpy sensing device for use in sensing both outdoor and return air in an air conditioning control system.
It is another object of this invention to provide a sensing device for sampled air which senses the wet bulb temperature of the air under conditions which approximate adiabatic saturation.
It is a general object of this invention to overcome the disadvantages of the traditional wick-type wet bulb instrument and to provide a unique and simple control scheme for the control of conventional comfort air conditioning systems as well as constant temperature and humidity systems utilizing air washers.
These and other objects of this invention will become apparent from a review of the following written description of the invention taken in conjunction with the accompanying drawings.